Modern computer systems include at least one microprocessor or other similar controlling device, and memory, commonly including several levels of cache memory. One type of cache memory is located on the same die as the microprocessor and is referred to as Level 1 or “L1” cache. Typically, the L1 cache includes a very small amount of memory but can be accessed very quickly by the microprocessor due to its proximity to the microprocessor. Another type of cache, known as Level 2 or “L2” cache, is normally mounted in the same package as the microprocessor but on a different die. L2 cache typically includes more memory than the L1 cache but cannot be accessed as quickly. A third level of cache, known as Level 3 or “L3” cache, is normally mounted on a separate circuit board or circuit card from the microprocessor package and typically includes more memory than the L1 and L2 caches. However, the L3 cache cannot be accessed as quickly as the L1 and L2 caches, due to its greater distance from the microprocessor.
FIG. 1 shows an example of an L3 cache card design. An L3 cache die can be assembled in a wire-bond or flip-chip type of package, and the package can be surface-mounted on a printed circuit board (PCB) card. Alternatively, the L3-cache die can be directly mounted on a PCB card with or without encapsulate. Hence, the illustrated design includes a PCB card 1, on which one or more cache memory dies (or packages containing such dies) 2 are mounted, and edge fingers 3 to allow the PCB card to be inserted into an edge connector on a motherboard.
FIG. 2 shows how the L3 cache card of FIG. 1 can be installed in a computer system. The L3 cache card 1 can be inserted into an edge connector 21 that is mounted on the motherboard 22, a shown. Also mounted on the motherboard 22 is a socket 23, which allows the microprocessor package 24 to be coupled to the motherboard 22. A heat sink 25 is thermally coupled to the microprocessor package through a package heat spreader 26.
The design of FIGS. 1 and 2 has several disadvantages. First, the distance between the microprocessor package and the L3 cache is undesirably large, which increases the L3 cache access time. The L3 cache card also consumes additional space and requires edge connectors on the motherboard. Further, the L3 cache card tends to block some of the airflow through the heat sink, hindering the cooling of other components, such as microprocessor, chipset, and voltage regulators.
An alternative approach is to mount the L3 cache on the same microprocessor package, like a multi-chip module (MCM). However, the cost of such an approach is very high and manufacturing is difficult.